This invention relates to methods for welding galvanized material and more particularly to methods for welding galvanized material without disruption of weld pools by vapor pressure generated by a zinc rich coating at a weld joint.
One problem in the welding of galvanized material arises because the zinc rich coating has a higher vapor pressure than the material of the mild steel onto which the galvanized coating is applied. Molten weld pools can be disrupted by the high vapor pressure which is generated when such coating is melted during the welding process. Consequently, the weld zone may be distorted resulting in poor quality welds.
U.S. Pat. Nos. 3,881,084 and 3,969,604 disclose a method for welding sheet metal material having a galvanized coating. The methods use electron or laser beams which penetrate the pieces and the problem of vapor pressure disruption is purportedly solved by alloying a substance with the zinc rich coating to produce a lower vapor pressure which will not adversely affect the quality of the weld. The '084 patent provides an iron oxide coating at the galvanized joint to produce vapor pressure reduction.
U.S. Pat. No. 4,661,677 discloses a deep penetration laser welding process that forms a patterned seam to join two separate parts. The parts are not galvanized and do not have the vapor pressure disruption problem solved by the present invention.
U.S. Pat. No. 4,642,446 discloses a method for laser welding a lap joint between galvanized sheet metal parts. The method uses a nickel coating to reduce the vapor pressure of the galvanized coat to thereby solve the problem of weld disruption.
Other known process include the provision of shims between first and second galvanized sheets to form a gap therebetween for the escape of the high vapor pressure produced by the galvanized coating during the weld process. Such shimming methods introduce a space between the material interfaces to define an alternative escape route for vapor pressure which would otherwise disrupt the weld pool. The problem with such an arrangement is that the space between the shimmed parts must be held constant or near constant to assure that the weld beam will be precisely positioned to form uniform welds at the sheets to be joined. In continuous processes the tolerance requirements to achieve such results are extremely tight and as a practical matter are difficult to maintain.